Expansion vessel replacement?

Hi all,

I have a Glowworm 80e combi with pressure issues.

Low pressure on the gauge (sub 0.5bar). Filling loop used to increase pressure. Turn heating on and working, heating off and pressure drops below level it was increased to.

Did some testing and when the heating is turned on, pressure increases. If turned off before 3bar then returns to original pressure but if reaches 3bar, the PRV kicks in and when turns off pressure drops below original value (longer the PRV on, the lower the final pressure).

So from searching about, it looks like as a blown expansion vessel. The air valve on the expansion vessel spurts out water also.

Any other possibility or should I fit a new expansion vessel?
Can I just fit an extra vessel by teeing of the heating loop and if so, does it matter where?
And what happens to the old vessel - does it stay as part of the circuit and remain filled with water?

Hope you can help....

I think it's an entirely separate issue but..... I have noticed a leak from the DHW flow switch (the front cover gasket).